1. Field of the Invention
This invention relates to a method of coextracting neptunium and plutonium, and more particularly to a method of performing the coextraction by exposing a nitric acid solution containing neptunium and plutonium to ultraviolet radiation to control their valences.
2. Description of the Relates Art
In the reprocessing of spent nuclear fuels, high-radioactive waste produced in a nuclear reactor is removed after co-decontaminating and distributing processes. The distributing process is a process for separating uranium and plutonium from each other after extracted from the dilution solution of HNO.sub.3 in the co-decontaminating process. The refining cycle is a process to remove contaminating FP elements from uranium and plutonium which are separated in the distributing process. The impurities in uranium are chiefly plutonium and neptunium. On the other hand, plutonium contains uranium and neptunium as main impurities. The refining cycle is a process for removing these impurities.
After distributing process, Pu.sup.3+ in the aqueous phase is oxidized to Pu.sup.4+ and this Pu.sup.4+ is extracted in the organic phase, whereupon a dilute HNO.sub.3 is added to the extracted Pu.sup.4+ to reversely back-extract it into the aqueous phase, recovering plutonium. In this refining cycle, it has been customary to use nitrate (nitrogen dioxide NO.sub.2 in particular) as an oxidizing agent, and hydroxylamine nitrate NH.sub.3 OHNO.sub.2 (HAN) is used as a reducing agent. The oxidizing agent and the reducing agent as well as a stabilizing agent to be added as needed are selected since they will produce a small quantity of radioactive secondary waste.
However, in the case where nitrogen oxide is used as an oxidizing agent, although small amount of radioactive secondary waste substance would be produced, a large quantity of process waste fluid of oxidizing agent would be produced. Another problem with this case is that the TBP (tributylphosphate) solvent would be degraded.
This would be a problem not only in oxidizing Plutonium (Pu) but also in oxidizing neptunium (Np); for example, it would be a problem in converting Np(V), which is unable to be extracted in the organic phase, into Np(VI), which is able to be extracted in the organic phase.
In addition, for producing mixed nuclear fuel containing plutonium and neptunium, a method which can coextract them easily has been in keen demand in the art.